The present invention is directed to a process for removing and dissolving zirconium oxide and/or titanium oxide containing sand mold residues on cast parts in salt melts.
Complicatedly shaped structural parts for the construction of machines or apparatus are frequently produced of cast steel by means of precision casting processes. Besides the customary sand molds which usually consist of pure quartz or quartz-aluminum oxide mixtures, there are also used zirconium oxide, titanium oxide or other metal oxide for improving the accuracy of dimensions and for creating a clean, smooth surface in the production of the casting molds. Zirconium oxide containing molding sands have proven to be particularly good and are much used in the fine casting art.
Especially with complicated shaped cast parts there is the problem that after breaking of the casting mold into pieces residues to the molding sand still adhere very firmly to the surface of the embedded cast piece and especially can only be removed with difficulty from hollow spaces, complex shaped cavities or bores.
To remove this molding sand in the compressed state, one skilled in the art normally uses mechanical and chemical cleansing processes.
Generally molding sand residues are preliminarily cleansed with hammer and chisel, with Widia-borers, by scraping, abrading or scouring by hand, the final cleansing takes place after that usually by repeated sand or ball blasting or using different types of blasting processes (centrifugal blasting, wet cleaning) or blasting medium (steel shot blasting). These mechanical processes have several disadvantages. They produce high operating costs, since they require a lot of work and pay for the workers and they have health risks since there is a danger of silicosis. As further disadvantages mention is made of the high wear and tear on the work tools (e.g. Widia borers, grinding wheels, etc.) and the high waste which particularly arises through rounding off the edges in the blasting process. Besides especially complicated sand mold particles generally cannot be removed mechanically under certain circumstances.
Where the employment of mechanically cleansing methods fail to work, e.g. in deep hollow spaces, bent or very fine bores, etc. chemical methods are used.
Additionally, the cast parts are frequently treated over many hours, indeed days, in hot aqueous sodium hydroxide or with aqueous hydrofluoric acid (e.g. Giesserei 66(1979), 406 or Fonderie 227 (1965) 29).
Furthermore, there are known processes in which the cast pieces to be cleansed are dipped in salt melts and desnaded electrochemically (Werkstoff und Betrieb 107(1974)101).
The known chemical methods fail to work particularly if there are included difficultly soluble chemical inert metal oxides such as zirconium oxide or titanium oxide in the molding sand. Such molding sands, however, are precisely those frequently used in precision casting. One is then generally completely directed to the mechanical cleansing.
The previously used cleansing processes in the casting art therefore are indicated in the technical literature as in general still needing great improvement.
Therefore it was the problem of the present invention to find a process for removal of sand mold residues, particularly those containing zirconium oxide (zirconium dioxide) and/or titanium oxide (titanium dioxide), on cast parts in salt melts with which there can be removed even difficulty soluble, chemically inert metal oxides without applying an electrical current.